Coated paper suitable for stylus inscription and method of making the same



March 27, 1956 RosENTHAL 2,739,909

COATED PAPER TABLE FOR STYLUS TNS PTToN AND METHOD OF' MAKIN HE SAMFiled June 29, 50

United States Patent O COATED PAPER SUITABLE FOR STYLUS IN- gRMIllTIONAND METHOD 0F MAKING THE Fritz Rosenthal, Nashua, N.- H., assigner toNashua Corporation, a corporation of Massachusetts Application June 29,1950, Serial No. 171,125 12 Claims. (Cl. 117-161) This invention relatesto sheet material (which usually will be paper and is so termed herein)having a coating thereon normally substantially opaque, but the physicalnature of which will be so changed under the localized action of astylus that it becomes more or less translucent,

providing a conspicuously contrasting mark. The contrast arises becauseof the local disclosure of the contrasting color of the underlyingpaper, usually and conveniently due to the presence of a dark coloredcoating layer initially applied thereto and over which thestylussensitive coating is applied. The obscuring coating film proposedto coat chart paper with a layer of discrete crystalsadapted to softenand coalesce to form a homogeneous translucent mass at the area affectedby the heated stylus. It has also been proposed to use as the obscuringcoating a blushed lacquer. Thus, for example, cellulose acetate may bedissolved in a solvent mixture of acetone and water and coated on apaper base. On drying the acetone evaporates more quickly, disturbingthe solvent balance and causing cellulose acetate particles toprecipitate out, the resultant film coating being tnus blushed as it istermed, and inthis application appearing as a white, opaque coating.Under a heated stylus the particles soften and coalesce to provide atranslucent homogeneous coating of the resin at the area affected by thestylus. It is difficult to obtain uniform results in utilizing suchlacquers because humidity conditions during manufacture affect theevaporation rate on which depends the iinal opacity and whiteness of theobscuring lm. This situation imposes a severe restriction on thecommercial use of blushed lacquers for this purpose.

In each of these cases the coating layer is opaque and hides the darkbackground beneath because it embodies a multiplicity of smallparticles, each homogeneous in itself, but having its own reflectingsurfaces. They may be compared to the solid pigment particles in a paintto which the hiding power of the paint is due. Or we might compare themto a layer of sand grains poured over the surface and covering the same.If it werel possible to visualize a suiiiciently intense yet localizedsource of heat effective locally to fuse such grains into clear,fused-silica glass, the analogy would be complete. The coating which Iam about to describe is physically quite different and operates ondifferent principles as will appear.

The invention will be explained with the aid of illustrative examples inthe accompanying specification. To this specification are annexeddrawings which, as will be well understood, are of an unrealistic anddiagrammatic nature, and probably will be of use chiey as a sort of f.ICC

graphical memorandum of the subject matter for the convenience of thosewho may have occasion hereafter to make use of the patent. In thesedrawings:

Fig. I is a fragmentary side elevation of a recording instrument havinga heated stylus;

Fig. 2 is a greatly enlarged perspective view partly in section of'asmall piece of a chart paper illustrative of the invention; and

Fig. 3l is a sectional view, much enlarged, schematically illustratingthe physical structure of the coating layer.

ln Fig. l I have illustrated a recording instrument having' amovable arm10 which is moved responsively to the variable condition which is beingmeasured and which has a stylus 12 electrically heated by currentsupplied from theI current source diagrammatically indicated at 14. Thestylus is shown as making contact with a paper chart C of the type aboutto be described, supported on the dial plate 16.

Referring to Fig. 2, the chart paper C may embody the paper base` 18which might in itself be of a dark co1- ored nature', but herein isshown as having a dark colored coated layer 20 applied thereto inaccordance with the usual practices of the paper art. Over this isapplied the coating film 22 characteristic of the invention, normallyopaque, but adapted to become locally translucent under the action ofthe stylus to reveal the dark colored layer underneath. Reference tothis coating layer 22 as a thinV filmV refers to a coating of suchthinness as is utilized in so-called coated papers. This is usuallyexpressed in termsl of pounds of coating material per ream of 3,000square feet'. The coatingis ordinarily only a mil or a fraction of a milin thickness. A coating over three mils in thickness would not beconsidered a thin film in this connection.

It isbelieved that an understanding of the invention will be facilitatedby a brief sketch at this point which should not be understood as eithera complete exposition thereof or as a definition of its scope, theformer being provided by the specification as a whole and the latter bythe annexed claims when read and understood in the light of thespeciiication. Brieliy it may be said thatthe film 22 consists of aheat-sensitive plastic orplastic composition whichhas athree-dimensional cellular structute including a multiplicity ofenclosed microscopic or submicroscopic voids distributed throughout -itsvolume beneath the outer surface thereof. The plastic, apart from thesevoids, is substantially continuous and homogeneous and the film as awhole is opaque because ofl its heterogeneous physical structure, due tosuch voids. Under the localized action of the stylus the plasticmaterial is fused, that is, it softens and coalesces with attendantcollapse of such voids and release of air therefrom: with the resultantproduction of a homogeneous, relatively transparent area disclosing theunderlying dark surface.

Such alm may be prepared and applied to the paper baseby applyingthereto an emulsion of the water-in-oiltype, wherein a film-formingplastic is the continuous phase and the dispersed phase is present inthe form of multitudinous droplets, at least almost all of which are ofmicroscopic or` submicroscopic dimensions, and by drying the film insuch manner that the dispersed phase is evaporated without essentialdisruption or substantial collapse of the cellular structure of thecontinuous phase. While the gelation of the plastic iilm and theevaporation of the water therefrom may to a certain extent besimultaneous, in general they occur in substantial sequence in thatorder, in that the plastic layer first attains such a degree ofsemi-solidity as to be effective to trap the dispersed water droplets.The water is then evaporated by diffusion through the rigid, orsubstantially rigid, cellular walls ofl the plastic and is replaced byair forming the voids already referred to. Thus, referring to Fig. 3,this might be considered a schematic view of an emulsion with thesectioned portion representing the continuous phase and the unsectionedcircles the droplets of the dispersed phase. It is to be assumed thatthe physical structure of the emulsion will not be exactly reproducedduring drying. There will be a certain shrinkage of the plastic layer.Certain of the droplets may merge and some may escape from the mass, butin general the solidied layer will reproduce in substantial degree insolid form the structure of the emulsion and the sectioned portion inFig. 3 represents the cellular layer of rigidied plastic and theunsectioned areas the multiplicity of included air-containing voids. Itmay be desirable to recall at this point that the vertical width shownin Fig. 3 represents a dimension which in practice may be only afraction of a mil.

The cellular character of the coating layer 22 and the presence thereonof the isolated voids may be strikingly demonstrated by submerging thepaper in a solvent for the resinous material. A substantial ebullitionof bubbles 'through the solvent is noted as the layer is dissolved.

This would not be the case if the material were merely porous with openpores defined by a multiplicity of aggregated particles, since there thesolvent would enter these pores and displace the air therefrom in majordegree as the material was lowered into the solvent.

As has already been explained, under the stylus the lm of resinousmaterial is caused locally to coalesce into a homogeneous mass withcollapse of the voids at the areas affected and release of airtherefrom. Such coalescence is dependent upon heat, pressure and time.In practical applications the time interval will usually be brief. Theheat should not be enough to ignite the chart or otherwise impair itsintegrity. In the case of recording instruments the pressure involved islight. If a localized beam of radiant heat were used the pressure factorwould be zero. lf the cellular coating were suiciently plastic it mightbe termed pressure-sensitive and would be adapted for inscription atroom temperature by pressure alone. In this case the heat factor is thenormal ambient temperature, as will be apparent when we consider thefact that we would not expect satisfactory results from the samepressure at sub-zero temperatures when the lm had thereby beensubstantially increased in rigidity.

The continuous phase of the emulsion may be termed a lacquer in that itis a hlm-forming material dissolved in a solvent adapted, on normalevaporation of such solvent, to dry down as a continuous layer. In thepresent instance the solvent is characterized by insolubility in water.The dispersed phase is aqueous, either water, a water-soluble liquid ora water-dissolved solid, incompatible with the ingredients of thecontinuous phase, both nlm-former and solvent, or a combination of waterwith such liquid or solid. The formation of an emulsion is aided by theaddition of an emulsifying assistant suitable for water-in-oildispersions, such as are known to the art, among which may be instancedstearyl dimethyl benzyl ammonium chloride, available under thecommercial name Triton K-60, aromatic polyglycol ethers, such as thatavailable under the commercial name Antarox A-200 or sorbitan trioleate,available under the commercial name Span 35. These emulsiiiers aresoluble in the lacquer solvent, which may be benzene, toluene,chloroform or any other meeting the requirements outlined above. Ethylcellulose has been found excellent for use as hlm-former, because of itsrange of heat-sensitivity, as well as because of its compatibility withmany substances capable of modifying the desired coating. Suchmodification may involve providing a lower softening temperature of thecoating which may be achieved by adding a plasticizer to thefilm-former. In addition to compatibility with the resin, theplasticizer should be incompatible withthe dispersed phase. Tricresylphosphate, dioctyl phthalate and castor oil are examples of plasticizerswhich ll these requirements and which I have successfully used. Otheradditives, which are known to enhance gloss and hardness of ethylcellulose coatings, may be used to advantage. For example, I haveobtained such results by incorporating a chlorinated diphenyl resincommercially known as Aroclor 5460 in proportions ranging from 0 to 75%by weight of ethyl cellulose. Coumarone-indene resins are also useful asresin extenders.

Satisfactory white coating are obtained when the evaporation of thedispersed phase occurs essentially after the evaporation of the lacquersolvent, i. e. after gelation of the lacquer ilm. When a solvent ofrelatively low evaporation rate is used, such as toluene or even xylene,it may be advantageous to retard the evaporation rate of the water orother components of the dispersed phase. This can be effected by addingto water numerous water-soluble substances, for example salts such assodium acetate or lead acetate, but especially polyvalent alcohols, suchas ethylene glycol or glycerol. Such alcohols are insoluble in thelacquer solvent and incompatible with the filmformers.

Emulsions thus formed are applied to a suitable base paper previouslycoated with a black or dark-colored base coating as known in the art.The emulsion is applied at such a rate as to form a thin coating (3 to 5pounds, or even less, per ream of paper). It is possible in accordancewith the invention to provide coatings of such order of weight having,when coated over a black backing a whiteness, measured by a Photometeras high as 86% of a magnesium carbonate standard.

One of the important features of this invention is the wide range overwhich the ratio of lacquer solution to dispersed phase may be varied ina water-in-oil emulsion. While in a blushed lacquer the ratio of solventto water is limited by the water tolerance of the lacquer (in thevicinity of 30 parts water to 100 parts lacquer solution), it ispossible in the emulsion system to increase the water (dispersed phase)to lacquer ratio to :100 if desired. It is thus possible to alter atwill the characteristics of the emulsion and of the heat-sensitivecoating, which may be prepared from the emulsion.

More specifically, the dispersed phase determines the degree of aerationof the nal coating in two Ways, (l) the water to lacquer ratio asindicated in the previous paragraph and (2) the particle size of thedispersion which may be reduced by such mechanical means as a passthrough a homogenizer. Such homogenization rends the emulsion morestable and thus improves the degree of aeration of the solidifiedcoating resultant from evaporation of the dispersed phase. This degreeof aeration determines the whiteness of the final coating. Aeration isalso mainfested by a measurable increase in apparent volume of thecoating.

An important practical advantage of these coatings is that the degree ofaeration can be controlled, as l have outlined, by the amount of thedispersed phase and the nature of the dispersion. Aerated resins ofdetermined physical properties are reproducible to a degree practicallyimpossible with blushed lacquers of the prior art, which depend on suchoutside factors as temperature and relative humidity.

I shall next give numerous examples which will further elucidate thenature of the invention and in the light of which and of theexplanations already given and to be given those skilled in the art willbe enabled, Without experimentation of an inventive character, to applythe principles of the invention to the selection and compounding ofother specific ingredients.

Ethyl cellulose is a very desirable plastic for use as the solid portionof the cellular film. The tirst eight examples now to be givenillustrate the use of this mate rial in various ways. in all these casesthe uniformity and stability of the emulsions are enhancedr by passingthem through a homogenizer.

Example l 41.25 grams of ethyl cellulose, grade N-l are dissolved in 650cc. of toluene under stirring. Grade N-100 identifies a resin, asolution of which in a solvent consisting of 80 parts toluene and 20parts ethanol has a viscosity of 93.5 centipoises. As emulsier, 1.9grams of Triton K-GG are added. Under constant high-speed stirring, 92cc. of distilled water are added dropwise or in the forni of a tinespray at a rate slow enough to permit uniform emulsitication. Thisemulsion produces a photometer reading of 73% when applied and dried asa coating of 6.0` pounds per ream. The

lapparent density of the coating isy measured to be 0.39.

Example 2 This differs from Example 1 in the amount of water used toform the dispersed phase, 184 cc. of water, or twice as much as inExample l. This emulsion produces a photometer reading of 80% whenapplied and dried as a coating of 4.0 pounds per ream. That is, thecoating is lighter in Example 1, but the whiteness is superior. Theapparent density ofthe coating is measured to be 0.16.

Example 3 This is like Example 1 except a still larger amount of wateris used, 504 cc. This emulsion produces a photometer reading of 71% whenapplied and dried as a coating of 2.2 pounds per ream. The apparentdensity of the coating is measured to be 0.08.

Example 4 This example illustrates the use of a less volatile solventfor the resin and an evaporation retarder for the aqueous constitutent.if the resin solvent is less readily driven oil the continuous resinphase does not pass over into the solid form so rapidly and it might bepossible for much of the water to evaporate before it was trapped, withresultant collapse to a considerable degree ofthe cellular structure andthe production of a coating of poor hiding power. This may be avoided bythe additionv to the water phase of an ingredient retarding itsevaporation that is, providing a decreased vapor pressure of thedispersed phase. This is the specific example.

41.25 grams of ethyl cellulose, grade kN-100, are dissolved in 650 cc.of xylene under stirring. As emulsitier, 1.9 grams of Triton isi-60 areadded. Under constant high-speed stirring, a mixture of 378 grams ofethylene glycol and. 126 cc. or distilled water are added dropwise or inthe form of a tine spray at a rate slow enough to permit uniformemulsitication. This emulsion produces a photometer reading of 71% whenapplied and dried as a coating of 2.2 pounds per ream.

Example 5 This example as contrasted with Example 2 illustrates the useof a smaller amount of resin of greater molecular weight and hencegreater viscosity, whereby' a lower proportion of solids may be utilizedto provide a coating of acceptable whiteness and opacity. Ethylcellulose of grade N-300, having a viscosity of 319.7 centipoises ascontrasted with 93.5 for grade N-100 previously Example 6 This. example,as contrastedwith Example 2, illustrates theadditionof a plasticizer inorder to reducev the soften Examples 7 and 8 Examples 7 and 8 areexamples in which a mixture of. resinous plastics is utilized as thelacquer base, or otherwise expressed, ethyl cellulose as used in theprevious examples is modified by added resins. In these instancescoatings of improved gloss and hardness and a superior degree ofwhiteness are provided for.

Example 7 41.25 grams of ethyl cellulose, grade N-lOO, are dissolved in650 cc. of toluene under stirring. As resin modiiier 10.25 grams ofAroclor 5460" and as emulsiiier, 1.9 grams of Antarox A-200 are added.Under constant stirring, 229 cc. of distilled water are added dropwiseor in the form of a fine spray at a rate slow enough to permit uniformemulsication. This emulsion produces a photometer reading of 83%y whenapplied and dried as acoating of 5.4 pounds per ream.

Example 8 41.25 grams of ethyl cellulose, grade N-l00, are dissolved in650 cc. of toluene under stirring. As resin modier, 41.25 grams of acoumarine-indene resin and as emulsitier, 1.9 grams of Antarox A-200 areadded. Under constant stirring, 620 cc. of water are added drop- Wise orin the form of a fine spray at a rate slow enough to permit uniformemulsiication. This emulsion produces aphotometer reading of 82% whenapplied and dried as a coating of 5.8 poundstper ream.

I have given a series of examples in which ethyl cellulose is used asthe resin. I will here give other examples involving the use of otherthermoplastic polymer materials. These have been selected as a typicalcross-section of high polymer thermoplastic resins. l shall give only asingle example for each resin, as the possibilities of variation will beapparent from the examples already given.

Example 9 40 grams of polystyrene are dissolved in 200 cc. of benzeneunder stirring. As emulsifier, 2 grams of sorbitan sesquioleate,available under the commercial name Arlacel C, are added. Under constantstirring, cc. of water are added dropwise or in the form of a ne sprayat a rate slow enough to permit uniform emulsication. This emulsionproduces a photometer reading of 74% when applied and dried as a coatingof 5.8 pounds per ream.

Example 10 8 grams of polyvinylchloride-acetate (commercial grade VYHHof Carbide and Carbon Chemicals Corporation) are dissolved in 60 cc. ofpropylene dichloride under ystirring. As emulsiier, 0.5 gram of ArlacelC"y are added. Under constant stirring, 30 cc. of water are added at arate slow enough to permit uniform emulsification. This emulsionproduces a photometer reading of 69% when applied and dried as a coatingof 5.9 pounds per ream.

Example l] 25 grams of polymethyl methacrylate, having a molecularweight of 31,600 or more and a ow temperature (Max.) of 138 C. aredissolved in a mixture of 100 cc.

of toluene and 50 cc. of methylene dichloride. As emulsifier, 2 grams ofArlacel C are added. Under constant stirring, 75 cc. of water are addedat a rate slow enough to permit uniform emulsication. This emulsionproduces a photometer reading of 82% when applied and dried as a coatingof 10.5 pounds per ream.

Example 12 25 grams of nitrocellulose, of a grade having a viscosity of-6.5 seconds in a solution of 12.2% concentration, are dissolved in 200cc. of n-butyl acetate. As emulsier, 2 grams of an anhydrous aliphaticpolyethylene glycol amide, available under the commercial name AntaroxG-l00, are added. Under constant stirring, 200 cc. of water are added ata rate slow enough to permit uniform emulsication. This emulsionproduces a photometer reading of 77% when applied and dried as a coatingof 4.3 pounds per ream. i

Example 13 l0 grams of cellulose acetate of a grade having a low acetylvalue and an Eastman viscosity of 2-5 seconds are dissolved in a mixtureof 100 ce. of methylene dichloride and l0 cc. of isoeoctyl alcohol understirring. As emulsier, 1.5 grams of Antarox G400 are added. Underconstant stirring, 55 cc. of water are added at a rate slow enough topermit uniform emulsiiication. This emulsion produces a photometerreading of 77% when applied and dried as a coating of 5.5 pounds perream.

Example 14 I shall now discuss in connection with a specilic example,the use of a low polymer resin, not because the example provides aparticularly advantageous application of the principles of theinvention, but because of the light it sheds on such principles. By wayof example l have chosen a coumarone-indene resinfhaving a melting pointof about 150 C. of a type known by the commercial designation NevindeneR-3. This resin being of low molecular weight has a rather lowviscosity, also it tends to be brittle, a property also inherent inresinous materials of low molecular weight. l'n accordance with theinvention, as already explained, entrapment of the water droplets andresultant whiteness in the solidified layer are achieved by postponingsubstantial evaporation of the Water until a certain degree of viscosityor rigidity has developed in the continuous phase, due to solventevaporation. Thus in a resin of low molecular weight such as thecournarone-indene resin referred to, this rigidity is reached only at amore advanced degree of solvent evaporation as compared with the highermolecular weight materials. Therefore to insure retention of waterdroplets sulhcient for desired whiteness a solvent of high volatilityshould be chosen. Thus a solution of coumarone-indene resin NevindeneR-3 in toluene, emulsified with an equal volume of water by means of anemulsifier Arlacel C did not produce a satisfactory coating, apparentlybecause the relatively high-boiling toluene maintained the lacquercomponent suiciently soft or fluid to permit the water of the dispersedphase to escape to the air. The same materials, utilizing benzene as asolvent, on the other hand produced a moderately good coating whenutilized in a relatively thick layer. Thus a coating of 2.21/2 poundsper ream gave a photometer reading of 52%. That is, as compared withExample l, about four times as much coating material was required toprovide a reflectance value only about two-thirds as great.

The qualities of such lower polymer coatings, however, may besubstantially improved by incorporating there with a proportion of highpolymer material. This is an example wherein the lacquer base is aplastic composition involving more than one resin, the resins havingdifferent names. Thus, a similar emulsion utilizing as the lacquer base97% coumarone-indene resin and 3% ethyl cellulose and laid down as acoating of 11.2 pounds per ream,

tra?

gave a Photometer reading of 54%. That is, equally good reflectance wasobtained with about 'half the material as compared with the unmodifiedresin. An increase in the proportion of high polymer resin used, forinstance say 10% of ethyl cellulose, results in a further improvement.Example 8, given above, is an example wherein 50% of coumarone-indeneresin is combined with 50% ethyl cellulose and the low viscosity resinin that case acts as a useful modifier of the ethyl cellulose to affordhighly desirable physical properties.

Example 15 I shall next give anA example of a coating which may beconsidered pressure-sensitive rather than heat-sensitive, that is, itwill yield an inscription under moderate pressure at room temperatures.lt will bev understood that for practical purposes the coating should oesuch as to resist smudging during handling. Thermoplastic materials ofrelatively low molecular weight will tend to be more pressure-sensitivethan homologues of higher molecular weight, and moreover, theincorporation of plasticizers to enhance pressure sensitivity isfacilitated in materials of lower molecular weight because of improvedcompatibility of such plasticizers therewith in comparison with thehomologues of the former of higher molecular Weight. Guided by theseconsiderations the specific example hereinafter set forth utilizes ethylcellulose grade N-lO, having in solution form as previously described aviscosity of 9.3 centipoises, substantially lower than grades N-lOO andN-300 which have been referred to in preceding examples.

The specific data follows. l0 grams of ethyl cellulose grade N-l0 aredissolved in 75 cc. of benzene under stirring. As plasticizer 5 grams oftricresyl phosphate, and as emulsilier 0.5 gram of Antarox A-200 areadded. Under constant stirring 75 cc. of water are added at a rate slowenough to permit uniform emulsication. This emulsion produces aphotometer reading of 70% when applied and dried as a coating of 4.9pounds per ream.

As a measure of its sensitivity to pressure a sapphire stylus having atop rounded to the curvature of M6 of an inch radius was pressedthereagainst with the force of one ounce. The coating became transparentunder this pressure, providing a clear marking.

It is apparent from the examples given that many different componentsmay be utilized in the manufacture of coatings under the invention. Alsonew synthetic resins and polymers are constantly being developed andmade commercially available, many of which undoubtedly will be foundadaptable as components of such coatings. It is therefore not onlyimpossible to attempt a comprehensive catalog of useful components, butto attempt to apprehend or describe the invention in its broader aspectsin terms of the chemical names of the components used would bemisleading. The invention is not chemical but physical, beingcharacterized by the physical form of the completed coating and theindividual composition of any ingredients is important only in the sensethat the individual properties of the elements of any mechanicalassemblage are important to their proper combination and coaction. Toformulate a specic recipe for a composition comparable to those in theexamples given will call for a certain familiarity of the physicalproperties of resins and other materials and their relativecompatibility but the ofiice of the chemist writing such a recipe wouldbe like that of a mechanical engineer who prescribes in the constructionof a machine the proper materials and the proper dimensions therefor,knowledge of the materials available, will know or deduce with condencetheir applicability for the purposes of the invention or, otherwise, andin the case of novel materials routine tests, not of an inventivenature, Will provide reliable data.

I am aware that the invention may be embodied in other specific formswithout departing from the spirit or One skilled in the art, from hisessential attributes thereof, and l therefore desire the presentembodiment to be considered in all respects as illustrative and notrestrictive, as is in fact clear in several v matters from thedescription itself. Reference is to be had to the appended claims toindicate those principles of the invention exemplified by the particularembodiment described and which I desire to secure by Letters Patent.

I claim:

1. Recording material suitable for receiving inscriptions from a styluscomprising a backing an adherent obscuring lm covering its surface whichlm consists of thermoplastic resinous material and has athree-dimensional cellular structure including a multiplicity ofdiscrete microscopic or sub-microscopic enclosed voids beneath the outersurface thereof and distributed throughout the volume of the film in amajor proportion of its thickness, the resinous film apart from saidvoids therein being substantially continuous and homogeneous, the filmbeing normally opaque throughout its area because of its heterogeneousphysical structure due to the inclusion of such voids, but adapted forlocal coalescence under the stylus to permit release of air from saidvoids and collapse thereof with resultant production of a homogeneous,relatively transparent area disclosing the underlying surface.

2. Recording material as defined in claim l, wherein the resinousmaterial consists essentially of ethyl cellulose.

3. Recording material as defined in claim 2, wherein a compatible resinis admixed with the ethyl cellulose to harden the film.

4. Recording material as defined in claim 1, wherein the resinousmaterial consists essentially of polystyrene resin.

5. Recording material as defined in claim 1, wherein the resinousmaterial consists essentially of polyvinyl resin.

6. Recording material as defined in claim 5, wherein the resinousmaterial consists essentially of polyvinyl chloride acetate.

7. Recording material as defined in claim l, wherein the resinousmaterial consists essentially of acrylate polymer.

8. Recording material as defined in claim 7, wherein the resinousmaterial consists essentially of methyl methacrylate.

9. Recording material as defined in claim l, wherein the resinousmaterial consists essentially of cellulose ester.

l0. A method of preparing recording material suitable for receivinginscriptions from a stylus and comprising a supporting backing having anobscuring film covering its surface: the method comprising dissolving ina volatile solvent therefor a thermoplastic, film-forming, resinousmaterial which in the form of a solid film is at least slowly perviousto the vapor of the liquid hereinafter referred to, and softenable underthe heat and pressure of the stylus;

emulsifying in said solution as a continuous phase a volaule liquidwhich is a non-solvent for the resinous material and is substantiallyincompatible with the solution, the liquid being present in significantquantity to provide as a dispersed phase multitudinous microscopic orsubmicroscopic droplets uniformly distributed at random throughout thevolume of the continuous phase; depositing a film of the emulsion as acontinuous superficial coating extending over the area of the backing,gelling the'resinous material by substantial evaporation of its solventand in substantial sequence to such gelling evaporating the disperseddroplets by diffusion through the essentially undisrupted anduncollapsed walls of the microscopically cellular structure provided bysuch gelled resinous material.

l1. The method of claim 10 when an ingredient insoluble in the volatilesolvent and incompatible with the resinous material is added to thevolatile liquid to decrease the vapor pressure thereof.

l2. Recording material suitable for receiving inscriptions by theapplication of heat thereto in a delimited portion of its areacorresponding in form to a signcant mark, said recording materialcomprising a backing and an adherent obscuring film of less than fifteenpounds per rearn covering its surface which film consists ofthermoplastic resinous material and has a three-dimensional cellularstructure including a multiplicity of discrete microscopic orsubmicroscopic enclosed voids beneath the outer surface thereof anddistributed throughout the volume of the iilm in a major proportion ofits thickness, the resinous film apart from said voids therein beingsubstantially continuous and homogeneous, the film being normally opaquethroughout its area because of its heterogeneous physical structure dueto the inclusion of said voids and having a reflectance value greaterthan the lm being adapted for local coalescence under application ofheat thereto in such a delimited portion of its area to permit therelease of air from the voids at said portion with collapse of saidvoids and resultant production of a homogeneous, relatively transparentmark visibly contrasting with the adjacent unaltered film surface.

References Cited in the le of this patent UNITED STATES PATENTS2,085,052 Taylor June 29, 1937 2,135,538 Sherman Nov. 8, 1938 2,185,046Voorhees Dec. 26, 1939 2,245,499 Reichel lune l0, 1941 2,256,483Johnston Sept. 23, 1941 2,299,991 Kallock Oct. 27, 1942 2,313,810 DaltonMar. 16, 1943 2,332,121 Trowell Oct. 19, 1943 2,346,957 Wuertz Apr. 18,1944 2,433,849 Lathrop Ian. 6, 1948 2,505,353 Fisk Apr. 2S, 19502,519,660 James Aug. 22, 1950

1. RECORDING MATERIAL SUITABLE FOR RECEIVING INSCRIPTIONS FROM A STYLUSCOMPRISING A BACKING AN ADHERENT OBSCURING FILM COVERING ITS SURFACEWHICH FILM CONSISTS OF THERMOSPLASTIC RESINOUS MATERIAL AND HAS ATHREE-DIMENSIONAL CELLULAR STRUCTURE INCLUDING A MULTIPLICITY OFDISCRETE MICROSCOPIC OR SUB-MICROSCOPIC ENCLOSED VOIDS BENEATH THE OUTERSURFACE THEREOF AND DISTRIBUTED THROUGHOUT THE VOLUME OF THE FILM IN AMAJOR PROPORTION OF ITS THICKNESS, THE RESINOUS FILM APART FROM SAIDVOIDS THEREIN BEING SUBSTANTIALLY CONTINUOUS AND HOMOGENEOUS, THE FILMBEING NORMALLY OPAQUE THROUGHOUT ITS AREA BECAUSE OF ITS HETEROGENEOUSPHYSICAL STRUCTURE DUE TO THE INCLUSION OF SUCH VOIDS, BUT ADAPTED FORLOCAL COALESCENCE UNDER THE STYLUS TO PERMIT RELEASE OF AIR FROM SAIDVOIDS AND COLLAPSE THEREOF WITH RESULTANT PRODUCTION OF A HOMOGENEOUS,RELATIVELY TRANSPARENT AREA DISCLOSING THE UNDERLYING SURFACE.